Imaging and projection systems are well known in the art. Examples of imaging and projecting systems include a head-up display (HUD) and window displays. These imaging and projecting systems produce a display image that is either natural or combined with an artificial image generated by an electronic system toward a person's eye. A head-up display is mounted on a platform or any type of mounting not attached to the viewer which is incapable of transparent viewing, meaning the display image is overlaid on the normally viewed scenery. The display image appears at a distance from the viewer, as if it is part of the normal scenery. The display image is typically bright and has a high contrast that can be distinguished from the normal scenery by use of a beam splitter. A beam splitter or optical combiner is an optical device (e.g., a semi-transparent mirror), which enables the superimposing, or combining of two images to one.
The prior art requires the use of a very powerful image source. An optical combiner is required in order to enhance the viewed image over the high surrounding ambient light during daytime. Usually special coatings, diachronic, holographic, narrow band selective or ND (neutral density) filters are also used.
During night time or in a dimmed environment, the image source needs to be capable of reducing its brightness considerably to avoid image blooming. In order to achieve a constant contrast ratio (CR, defined as the ratio between the brightness of a visual element and that of the surroundings), for any ambient light conditions, the Display Device (DD)'s brightness needs to be adjusted accordingly.
In prior art HUD optical systems the display device is typically a powerful CRT or LCD 10 with a strong backlight using light emitting source like LED. The amount of light such a display needs to reach is 6,000 to 10,000 FC (foot candles), depending on the combiner's reflection ratio. Coatings and filters may be used within some prior art optical systems. The minimum required contrast ratio (CR) that allows the viewer to see display image against bright environment is 1:1.2. A sub-image is provided by an emissive display device (DD) where it is generated by an electronic image generator.
The optical equation for the contrast ratio is:CR=Bd*R%/BbWhere:CR—Contrast Ratio 20Bd—Display BrightnessR %—Reflection factor of the combinerBb—Background Brightness seen by the viewer, typically ambient light*(1−R %)For best performance: the R % is higher as possible while the Bb is lower as required, the goal of the head up displays is to achieve a ratio of image brightness to ambient light as high as possible. A search of the prior art did not disclose any literature or patents that read directly on the claims of the instant invention. However, the following U.S. patents are considered related:
Pat. No.INVENTORISSUED8,427,395Seder29 Apr. 20137,123,418Weber17 Oct. 20066,952,312Weber 4 Oct. 20056,744,478Asakura 1 Jun. 20042005/0041297He24 Feb. 2005
The U.S. Pat. No. 8,427,395, discloses a system that projects graphics upon a transparent windscreen head-up display of a vehicle. The system includes a graphics projection system projecting light upon the transparent windscreen head-up display and the transparent windscreen head-up display emits light in a limited field of view based upon the projected light.
The U.S. Pat. No. 7,123,418 discloses a head-up display that a projection system and a window having a target area where a reflective polarizer is positioned to reflect light from the projection system to a viewing area. Light from the projection system is p-polarized and strikes exposed window surface(s) at an acute angle to reduce or eliminate multiple or “ghost” images. The reflective polarizer includes a multilayer stack with refractive indices of individual layers chosen to reflect p-polarized light substantially more than s-polarized light over a narrow angular range.
The U.S. Pat. No. 6,952,312 discloses a head-up display that includes a projection system and a window having a target area where a reflective polarizer is positioned to reflect light from the projection system to a viewing area. Light from the projection system is p-polarized and strikes exposed window surface(s) at an acute angle to reduce or eliminate multiple or “ghost” images. The acute angle is closely matched to a Brewster angle of the exposed window surface(s).
The U.S. Pat. No. 6,744,478 discloses a head-up display system comprising a transparent plate. A liquid crystal display generates a display light of information with a plane of polarization included relative to an image plane vertical axis. A first optical rotation layer is disposed on a first surface of the transparent plate, and receives the display light from the liquid crystal display and optically rotates the plane of polarization of the display light. A second optical rotation layer is between the image plane of the liquid crystal display and a second surface of the transparent plate, and optically rotates the plane of polarization of the display light and allows S-polarized light to emanate toward the transparent plate at Brewster's angle.
The 2005/0041297 patent discloses basic helmet-mounted and head-up display systems with emitting image source like CRT or LED with the additional use of CLC-Based electrically controlled reflection surface. The display brightness to the viewer is electrically controlled by the combiner's reflective surface. However, at bright ambient light, when the viewer needs to have highest contrast ratio, the combiner provided in this patent will increase the combiner's reflection thus block most of the outside scenery for the viewer.
For background purposes and indicative of the art to which the invention relates, reference may be made to the following remaining patents found in the patent search.
Pat. No.INVENTORISSUED8,339,710Torii25 Dec. 20127,777,960Freeman17 Aug. 20106,636,185Spitzer21 Oct. 20036,262,848Anderson17 Jul. 20012011/0227487Nichols22 Sep. 2011